Gene Mapping in Phages

Mapping genes in bacteriophage requires the application of the same principles as those applied to mapping genes in eukaryotic organisms (Chapter 7). Crosses are made between viruses that differ in two or more genes, and recombinant progeny phage are identified and counted. The proportion of recombinant progeny is then used to estimate the distances between the genes and their linear order on the chromosome.

In 1949, Alfred Hershey and Raquel Rotman examined rates of recombination between genes in two strains of the T2 bacteriophage that differed in plaque appearance and host range (the bacterial strains that the phages could infect). One strain was able to infect and lyse type B E. coli cells but not B/2 cells (normal host range, h+) and produced an abnormal plaque that was large with distinct borders (rr). The second strain was able to infect and lyse both B and B/2 cells (mutant host range, hr) and produced normal plaques that were small with fuzzy borders (r+).

Hershey and Rotman crossed the h+ rr and hr r+ strains of T2 by infecting type B E. coli cells with a mixture of the two strains. They used a high concentration of phages so that most cells could be simultaneously infected by both strains ( FIGURE 8.25). Homologous recombination occasionally took place between the chromosomes of the different strains, producing h+ r+ and hr rr chromosomes, which

8.25 Hershey and Rotman developed a technique for mapping viral genes. (Photo from G.S. Stent, Molecular Biology of Bacterial Viruses. Copyright © 1963 by W.H. Freeman and Company.)

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Question: How can we determine the position of a gene on a phage chromosome?

Infection of E. coli B

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